Effect of protein and steroidal osteotropic agents on differentiation and epidermal growth factor-mediated growth of the CFK1 osseous cell line

Bone morphogenic protein: an elixir for bone grafting--a review

Bone morphogenetic proteins (BMPs) are multifunctional growth factors that belong to the transforming growth factor beta superfamily. This literature review focuses on the molecular biology of BMPs, their mechanism of action, and subsequent applications. It also discusses uses of BMPs in the fields of dentistry and orthopedics, research on methods of delivering BMPs, and their role in tissue regeneration. BMP has positive effects on bone grafts, and their calculated and timely use with other growth factors can provide extraordinary results in fractured or nonhealing bones. Use of BMP introduces new applications in the field of implantology and bone grafting. This review touches on a few unknown facts about BMP and this ever-changing field of research to improve human life.

Amphiregulin Is a Novel Growth Factor Involved in Normal Bone Development and in the Cellular Response to Parathyroid Hormone Stimulation

Parathyroid hormone (PTH) is the major mediator of calcium homeostasis and bone remodeling and is now known to be an effective drug for osteoporosis treatment. Yet the mechanisms responsible for its functions in bone are largely unknown. Here we report that the expression of amphiregulin (AR), a member of the epidermal growth factor (EGF) family, is rapidly and highly up-regulated by PTH in several osteoblastic cell lines and bone tissues. Other osteotropic hormones (1alpha,25-dihydroxyvitamin D3 and prostaglandin E2) also strongly stimulate AR expression. We found all EGF-like ligands and their receptors are expressed in osteoblasts, but AR is the only member that is highly regulated by PTH. Functional studies demonstrated that although AR is a potent growth factor for preosteoblasts, it completely inhibits further differentiation. AR also strongly and quickly stimulated Akt and ERK phosphorylation and c-fos and c-jun expression in an EGF receptor-dependent manner. Moreover, AR null mice displayed significantly less tibial trabecular bone than wild-type mice. Taken together, we have identified a novel growth factor that is PTH-regulated and appears to have an important role in bone metabolism.

Antagonistic relationship between Dpp and EGFR signaling in Drosophila head patterning

The Drosophila eye field that gives rise to the visual system and dorsal head epidermis forms an unpaired anlage located in the dorsal head ectoderm. The eye field expresses and requires both Dpp and EGFR signaling for its development. As shown in previous studies, EGFR is required for cell maintenance in the developing visual system. Dpp initially switches on the early eye genes so and eya in the eye field. Consecutively, high levels of Dpp in the dorsal midline inhibit these genes and promote development of head epidermis. We show that Dpp negatively regulates EGFR signaling, thereby increasing the amount of cell death in the dorsal midline. By this mechanism, Dpp controls the formation of a bilateral visual system and indirectly modulates cell death, which is essential for normal head morphogenesis. Loss of either Dpp or its downstream target, Zen, abolishes head epidermis fate and leads to the misexpression of dp-ERK in the dorsal midline. The resulting morphological phenotype consists of cyclopia, reduction of cell death, and failure of head involution. Ectopic expression of activated EGFR inhibits the Dpp target race and thereby causes cyclopia and defective head involution. We discuss possible mechanisms of Dpp and EGFR interaction in the embryo.

Receptor tyrosine kinases inhibit bone morphogenetic protein-Smad responsive promoter activity and differentiation of murine MC3T3-E1 osteoblast-like cells

Growth factors such as fibroblast growth factor-2 (FGF-2) and epidermal growth factor (EGF) that activate extracellular signal-regulated kinases (ERKs) through receptor tyrosine kinases (RTKs) stimulate proliferation but suppress differentiation of osteoblasts. To study the mechanism of this inhibitory action of these growth factors on osteoblastic differentiation, we evaluated Smad1 transactivity in MC3T3-E1 osteoblast-like cells by reporters of promoter activity of mouse Smad6, an early response gene to bone morphogenetic proteins (BMPs). FGF-2 and EGF inhibited alkaline phosphatase activity and Smad6 promoter activity stimulated by BMP-2. Overexpression of constitutively active MEK by adenovirus mimicked, but that of dominant negative Ras or treatment with a MEK1 inhibitor, PD098059, reversed, the inhibitory effects of these growth factors on both activities. These effects are mediated by BMP-responsive elements (BMPREs) on Smad6 promoter, because an artificial reporter driven by three tandem BMPREs gave similar results, and these effects were all abolished when the BMPREs were mutated. RTK-ERK activation inhibited the promoter activity even when BMP signal was mediated by a mutant Smad1, which lacks phosphorylation sites by ERKs, or by a Smad1 fused to Gal4 DNA binding domain, which constitutively localizes in the nucleus. These results show that the RTK-Ras-ERK pathway suppresses BMP signal by interfering with Smad1 transactivity. Because direct phosphorylation of Smad1 by ERKs is not required for the inhibition, other transcriptional factors that are phosphorylated by ERKs might be involved in the regulation of osteoblastic differentiation by ERKs.

FRL-1, a member of the EGF-CFC family, is essential for neural differentiation in Xenopus early development

Recent studies indicate an essential role for the EGF-CFC family in vertebrate development, particularly in the regulation of nodal signaling. Biochemical evidence suggests that EGF-CFC genes can also activate certain cellular responses independently of nodal signaling. Here, we show that FRL-1, a Xenopus EGF-CFC gene, suppresses BMP signaling to regulate an early step in neural induction. Overexpression of FRL-1 in animal caps induced the early neural markers zic3, soxD and Xngnr-1, but not the pan-mesodermal marker Xbra or the dorsal mesodermal marker chordin. Furthermore, overexpression of FRL-1 suppressed the expression of the BMP-responsive genes, Xvent-1 and Xmsx-1, which are expressed in animal caps and induced by overexpressed BMP-4. Conversely, loss of function analysis using morpholino-antisense oligonucleotides against FRL-1 (FRL-1MO) showed that FRL-1 is required for neural development. FRL-1MO-injected embryos lacked neural structures but contained mesodermal tissue. It was suggested previously that expression of early neural genes that mark the start of neuralization is activated in the presumptive neuroectoderm of gastrulae. FRL-1MO also inhibited the expression of these genes in dorsal ectoderm, but did not affect the expression of chordin, which acts as a neural inducer from dorsal mesoderm. FRL-1MO also inhibited the expression of neural markers that were induced by chordin in animal caps, suggesting that FRL-1 enables the response to neural inducing signals in ectoderm. Furthermore, we showed that the activation of mitogen-activated protein kinase by FRL-1 is required for neural induction and BMP inhibition. Together, these results suggest that FRL-1 is essential in the establishment of the neural induction response.

Intracellular BMP signaling regulation in vertebrates: pathway or network?

Bone morphogenetic proteins (BMPs), members of the TGF-beta superfamily of secreted signaling molecules, have important functions in many biological contexts. They bind to specific serine/threonine kinase receptors, which transduce the signal to the nucleus through Smad proteins. The question of how BMPs can have such diverse effects while using the same canonical Smad pathway has recently come closer to an answer at the molecular level. Nuclear cofactors have been identified that cooperate with the Smads in regulating specific target genes depending on the cellular context. In addition, the pivotal role BMP signaling plays is underscored by the identification of factors that regulate members of this pathway at the cell surface, in the cytoplasm, and in the nucleus. Many of these factors are BMP-inducible and inhibit the BMP pathway, thus establishing negative feedback loops. Members of the BMP-Smad pathway can also physically interact with components of other signaling pathways to establish crosstalk. Finally, there is accumulating evidence that an alternative pathway involving MAP kinases can transduce BMP signals. The evidence and implications of these findings are discussed with an emphasis on early embryonic development of Xenopus and vertebrates.

Multiple roles of bone morphogenetic protein signaling in the regulation of cortical cell number and phenotype

Members of the bone morphogenetic protein (BMP) family have been implicated in multiple aspects of neural development in both the CNS and peripheral nervous system. BMP ligands and receptors, as well as the BMP antagonist noggin, are expressed in the developing cerebral cortex, making the BMPs likely candidates for regulating cortical development. To define the role of these factors in the developing cerebral cortex, we examined the effects of BMP2 and BMP4 on cortical cells in vitro. Cells were cultured from embryonic day 13 (E13) and E16 rat cerebral cortex in the absence or presence of different concentrations of fibroblast growth factor 2, a known regulator of cortical cell proliferation and differentiation. At E13, the BMPs promoted cell death and inhibited proliferation of cortical ventricular zone cells, resulting in the generation of fewer neurons and no glia. At E16, the effects of the BMPs were more complex. Concentrations of BMP2 in the range of 1-10 ng/ml promoted neuronal and astroglial differentiation and inhibited oligodendroglial differentiation, whereas 100 ng/ml BMP2 promoted cell death and inhibited proliferation. Addition of the BMP antagonist noggin promoted oligodendrogliogenesis in vitro, demonstrating that endogenous BMP signaling influences the differentiation of cortical cells in vitro. The distribution of BMP2 and noggin within the developing cortex suggests that local concentrations of ligands and antagonists define gradients of BMP signaling during corticogenesis. Together, these results support the hypothesis that the BMPs and their antagonist noggin co-regulate cortical cell fate and morphogenesis.

Bone morphogenetic protein-2 inhibits MAPK-dependent Elk-1 transactivation and DNA synthesis induced by EGF in mesangial cells

Bone morphogenetic protein-2 (BMP-2) is a member of the TGFbeta superfamily of growth and differentiation factors. We investigated the effect of BMP-2 on epidermal growth factor (EGF)-induced mitogenic signaling in kidney glomerular mesangial cells. BMP-2 dose-dependently inhibits EGF-induced DNA synthesis. Maximum effect was obtained at a concentration of 100 ng/ml. BMP-2 had no inhibitory effect on the EGF receptor (EGFR)-associated tyrosine kinase activity indicating that inhibition of DNA synthesis is due to regulation of post-receptor signaling event(s). EGF stimulates MAPK activity in mesangial cells in a time-dependent manner. Inhibition of MAPK by the MEK inhibitor PD098059 blocks EGF-induced DNA synthesis indicating the requirement of this enzyme activity in EGF-mediated mitogenic signaling. Furthermore, we show that exposure of mesangial cells to BMP-2 blocks EGF-induced MAPK activity which leads to phosphorylattion of Elk-1 transcription factor. Using a GAL-4 DNA binding-domain-Elk-1 transactivation domain fusion protein-based reporter assay, we demonstrate that BMP-2 inhibits EGF-induced Elk-1-mediated transcription. These data provide the first evidence that BMP-2 signaling in mesangial cells initiates a negative regulatory cross-talk with MAPK-based transcription to inhibit EGF-induced DNA synthesis.

Bone morphogenetic protein 2 inhibits platelet-derived growth factor-induced c-fos gene transcription and DNA synthesis in mesangial cells. Involvement of mitogen-activated protein kinase

Bone morphogenetic proteins (BMPs) play an important role in nephrogenesis. The biologic effect and mechanism of action of these proteins in the adult kidney has not yet been studied. We investigated the effect of BMP2, a member of these growth and differentiation factors, on mitogenic signal transduction pathways induced by platelet-derived growth factor (PDGF) in glomerular mesangial cells. PDGF is a growth and survival factor for these cells in vitro and in vivo. Incubation of mesangial cells with increasing concentrations of BMP2 inhibited PDGF-induced DNA synthesis in a dose-dependent manner with maximum inhibition at 250 ng/ml. Immune complex tyrosine kinase assay of PDGF receptor beta immunoprecipitates from lysates of mesangial cells treated with PDGF showed no inhibitory effect of BMP2 on PDGF receptor tyrosine phosphorylation. This indicates that the inhibition of DNA synthesis is likely due to postreceptor events. However, BMP2 significantly inhibited PDGF-stimulated mitogen-activated protein kinase (MAPK) activity that phosphorylates the Elk-1 transcription factor, a component of the ternary complex factor. Using a fusion protein-based reporter assay, we also show that BMP2 blocks PDGF-induced Elk-1-mediated transcription. Furthermore, we demonstrate that BMP2 inhibits PDGF-induced transcription of c-fos gene, a natural target of Elk-1 that normally forms a ternary complex that activates the serum response element of the c-fos gene. These data provide the first evidence that in mesangial cells, BMP2 signaling cross-talks with MAPK-based transcriptional events to inhibit PDGF-induced DNA synthesis. One target for this inhibition is the early response gene c-fos.

Opposite effects of FGF and BMP-4 on embryonic blood formation: roles of PV.1 and GATA-2

In adult vertebrates, fibroblast growth factor (FGF) synergizes with many hematopoietic cytokines to stimulate the proliferation of hematopoietic progenitors. In vertebrate development, the FGF signaling pathway is important in the formation of some derivatives of ventroposterior mesoderm. However, the function of FGF in the specification of the embryonic erythropoietic lineage has remained unclear. Here we address the role of FGF in the specification of the erythropoietic lineage in the Xenopus embryo. We report that ventral injection of embryonic FGF (eFGF) mRNA at as little as 10 pg at the four-cell stage suppresses ventral blood island (VBI) formation, whereas expression of the dominant negative form of the FGF receptor in the lateral mesoderm, where physiologically no blood tissue is formed, results in a dramatic expansion of the VBI. Similar results were observed in isolated ventral marginal zones and animal caps. Bone morphogenetic protein-4 (BMP-4) is known to induce erythropoiesis in the Xenopus embryo. Therefore, we examined how the BMP-4 and FGF signaling pathways might interact in the decision of ventral mesoderm to form blood. We observed that eFGF inhibits BMP-4-induced erythropoiesis by differentially regulating expression of the BMP-4 downstream effectors GATA-2 and PV.1. GATA-2, which stimulates erythropoiesis, is suppressed by FGF. PV.1, which we demonstrate to inhibit blood development, is enhanced by FGF. Additionally, PV.1 and GATA-2 negatively regulate transcription of each other. Thus, BMP-4 induces two transcription factors which have opposing effects on blood development. The FGF and BMP-4 signaling pathways interact to regulate the specification of the erythropoietic lineage.

Signaling specificity: the RTK/RAS/MAP kinase pathway in metazoans

The molecular basis by which commonly used signaling pathways are able to elicit tissue-specific responses in multicellular organisms is an important yet poorly understood problem. In this review, we use the receptor tyrosine kinase (RTK)/RAS/MAP kinase signaling cascade as a model to discuss various hypotheses that have been proposed to explain signaling specificity. Specificity can arise at the level of the receptor, through the modulation of signaling kinetics, through the interaction of different signaling pathways, and at the level of downstream signaling components. Mechanisms of specificity used by the RTK/RAS/MAP kinase signaling pathway might apply to other signaling pathways as well, and might help explain how multicellular organisms are able to generate tissues of diverse forms and functions from a small set of common signaling pathways.

Role of epidermal growth factor and its receptor in mechanical stress-induced differentiation of human periodontal ligament cells in vitro

The periodontal ligament (PDL) contains precursor cells for osteoblasts and cementoblasts. It has been shown that epidermal growth factor (EGF) inhibits dexamethasone-induced differentiation and up-regulates EGF-receptor (EGF-R) expression, whereas EGF-R is down-regulated in the course of differentiation. Thus it was suggested that EGF and its receptors act as a negative regulator of osteoblastic differentiation in PDL cells. In order to investigate further this hypothesis, human PDL cells were now used to elucidate the role of EGF and EGF-R in their proliferation and differentiation under mechanical stress-loaded conditions in vitro, as the PDL regularly receives mechanical stress from occlusal forces. As a model of mechanical stress, a cyclic stretch of 9 or 18% elongation was applied to the cells with a Flexercell cell-strain unit system. Alkaline phosphatase activity and osteocalcin mRNA expression were significantly induced by loading cyclic stretch for more than 4 days, whereas stretch slightly inhibited cell proliferation. Visualization of the actin stress fibres of the cells by rhodamine phalloidin revealed that approx. 10% of the total number of cells had become aligned perpendicularly to the direction of the stretch. The effects of stretch on alkaline phosphatase activity and cell proliferation were totally abolished by the presence of 10 ng/ml EGF. Western blotting of EGF-R protein demonstrated that stretch-induced differentiation accompanied the decreased expression of EGF-R protein in the cells. However, the amount of tyrosine-phosphorylated EGF-R upon EGF stimulation was restored to the control level in stretched cells. These results suggest that the EGF/EGF-R system acts as a negative regulator of differentiation of PDL cells regardless of the type of differentiation stimuli. Also, interaction between mechanical stress and the EGF/EGF-R system may participate in the osteoblastic differentiation of PDL cells and thereby regulate the source of cementoblasts and osteoblasts.

SMADs: mediators and regulators of TGF-beta signaling

The discovery of SMAD proteins has allowed the delineation of a mechanism by which TGF-beta and related growth factors convey their signals from membrane receptors all the way into the nucleus. SMADs are directly phosphorylated and activated by the receptors and then form heteromeric SMAD-SMAD complexes that move into the nucleus where they orchestrate transcriptional responses. In rapid succession, recent reports have identified different modes of SMAD regulation by phosphorylation and have defined the SMAD domains that mediate SMAD interactions, binding to DNA or transcriptional activation. The recent discovery of antagonistic SMADs and regulatory crosstalk with Ras/MAP-kinase pathways add to our rapidly expanding understanding of this major regulatory network.

Opposing BMP and EGF signalling pathways converge on the TGF-beta family mediator Smad1

The growth factor TGF-beta, bone morphogenetic proteins (BMPs) and related factors regulate cell proliferation, differentiation and apoptosis, controlling the development and maintenance of most tissues. Their signals are transmitted through the phosphorylation of the tumour-suppressor SMAD proteins by receptor protein serine/threonine kinases (RS/TKs), leading to the nuclear accumulation and transcriptional activity of SMAD proteins. Here we report that Smadl, which mediates BMP signals, is also a target of mitogenic growth-factor signalling through epidermal growth factor and hepatocyte growth factor receptor protein tyrosine kinases (RTKs). Phosphorylation occurs at specific serines within the region linking the inhibitory and effector domains of Smad1, and is catalysed by the Erk family of mitogen-activated protein kinases. In contrast to the BMP-stimulated phosphorylation of Smad1, which affects carboxy-terminal serines and induces nuclear accumulation of Smad1, Erk-mediated phosphorylation specifically inhibits the nuclear accumulation of Smad1. Thus, Smadl receives opposing regulatory inputs through RTKs and RS/TKs, and it is this balance that determines the level of Smad1 activity in the nucleus, and so possibly the role of Smad1 in the control of cell fate.

Characterization and cloning of a receptor for BMP-2 and BMP-4 from NIH 3T3 cells

The bone morphogenetic proteins (BMPs) are a group of transforming growth factor beta (TGF-beta)-related factors whose only receptor identified to date is the product of the daf-4 gene from Caenorhabditis elegans. Mouse embryonic NIH 3T3 fibroblasts display high-affinity 125I-BMP-4 binding sites. Binding assays are not possible with the isoform 125I-BMP-2 unless the positively charged N-terminal sequence is removed to create a modified BMP-2, 125I-DR-BMP-2. Cross-competition experiments reveal that BMP-2 and BMP-4 interact with the same binding sites. Affinity cross-linking assays show that both BMPs interact with cell surface proteins corresponding in size to the type I (57- to 62-kDa) and type II (75- to 82-kDa) receptor components for TGF-beta and activin. Using a PCR approach, we have cloned a cDNA from NIH 3T3 cells which encodes a novel member of the transmembrane serine/threonine kinase family most closely resembling the cloned type I receptors for TGF-beta and activin. Transient expression of this receptor in COS-7 cells leads to an increase in specific 125I-BMP-4 binding and the appearance of a major affinity-labeled product of approximately 64 kDa that can be labeled by either tracer. This receptor has been named BRK-1 in recognition of its ability to bind BMP-2 and BMP-4 and its receptor kinase structure. Although BRK-1 does not require cotransfection of a type II receptor in order to bind ligand in COS cells, complex formation between BRK-1 and the BMP type II receptor DAF-4 can be demonstrated when the two receptors are coexpressed, affinity labeled, and immunoprecipitated with antibodies to either receptor subunit. We conclude that BRK-1 is a putative BMP type I receptor capable of interacting with a known type II receptor for BMPs.

Effect of beta-alanyl-L-histidinato zinc on differentiation of osteoblastic MC3T3-E1 cells: increases in alkaline phosphatase activity and protein concentration

The effect of beta-alanyl-L-histidinato zinc (AHZ) on bone cell function was investigated in osteoblastic MC3T3-E1 cells. Cells were cultured for 3 days at 37 degrees C in a CO2 incubator in plastic dishes containing alpha-modified minimum essential medium supplemented with 10% fetal bovine serum. After the cultures, the medium was exchanged for that containing 0.1% bovine serum albumin plus AHZ (10(-7)-10(-5) M) or other reagents, and the cells were cultured further for appropriate periods of time. The presence of AHZ (10(-7)-10(-5) M) produced a remarkable increase of alkaline phosphatase activity and protein concentration in osteoblastic cells. Thus increase were seen with the prolonged cultivation (12-21 days). With the culture of 1, 3 and 12 days, the effect of AHZ (10(-6) M) to increase alkaline phosphatase activity and protein concentration was more intensive than the effect of zinc sulfate (10(-6) M). The AHZ effects were completely abolished by the presence of cycloheximide (10(-6) M), indicating that AHZ stimulates protein synthesis in the cells. The present study suggests that AHZ has a stimulatory effect on cell differentiation, and that this effect is partly involved on protein synthesis in osteoblastic cells.